This invention relates generally to a profile extrusion die and more particularly to a system and method for providing die swell or draw down (hereinafter die swell/draw down) information for designing a profile extrusion die.
Typically, a die designer uses experienced-based rules to derive a profile extrusion die design. After deriving the profile extrusion die design, the designer implements the design in an extrusion process. The designer then evaluates the design to determine if the die land length, die opening thickness, and die opening width produce the desired profile dimensions. If the dimensions do not have the desired profile dimensions, then the die is re-cut and implemented in another extrusion process. Usually there is many iterations of cutting and then changing the die before deriving a die design that yields the desired profile dimensions. This design methodology requires a lot of work and time and a tremendous amount of experience-based data. In addition, the designer must modify the die every time the dimensions change or when the extrusion process uses a new plastic resin. These situations also involve more time, work and money. As a result, the designer is reluctant to work with new plastic resins, which hinders the introduction of these resins into the profile extrusion market.
U.S. Pat. No. 5,608,637 discloses a semi-empirical die design methodology that enables a designer to readily design a die that shapes products having varying dimensions and products made from different types of resins. In U.S. Pat. No. 5,608,637, a designer uses a viscoelastic computer software package to provide a representation of a product made from a resin material and shaped by a die. The representation illustrates a relationship between die swell of the resin, shear rate of the resin through the die, and the ratio of die land length of the die to die opening thickness of the die. The designer uses the representation to determine instances where the ratio of die land length to die opening thickness minimizes die swell of the resin. The designer then studies the flow rate effect on die swell for experimental dies in an extrusion process and obtains experimental data therefrom. U.S. Pat. No. 5,608,637 arranges the representation and the experimental data into die design charts and the designer uses the charts to determine profile dimensions of a desired profile extrusion die.
A drawback with U.S. Pat. No. 5,608,637 is that the methodology provided in this patent uses an extruder to develop the die design charts for a particular resin. Using an extruder to develop die design charts is expensive and time consuming. Typically, the extruder needs about two to three gaylords of resin to generate a sufficient number of data points. In addition, the extruder has a tendency to introduce errors at different locations. For example, shear rate measurements and melt temperature control at the extruder die are questionable. The errors produced by the extruder result in the die design charts having inaccurate die swell/draw down values. Therefore, there is a need for a methodology that can provide die swell/draw down information for profile extrusion die design that is simpler, faster, and more accurate than the information provided by an extruder.